Method of cleaning metal strip



` March 11, 195s J, B, MURTLAND, JR 2,826,539

METHOD OF CLEANING METAL STRIP Filed DeO. 5. 1956 Fig.|

43' SPowelr up y es 1/ 5 \38 422 54 3s sa 4 Ind hl AAA v vv United tates METHOD F CLEANING METAL STRIP James B. Murtland, Jr., Tarentum, Pa., assignor to Allegheny Ludlum Steel Corporation, Breckenridge, Pa., a corporation of Pennsylvania Application December 5, 1956, Serial No. 626,506

8 Claims. (Cl. 204-141) atent titanium base alloys, zirconium and zirconium base alloys.

In producing strip metal of refractory metal of the class of titanium, titanium base alloys, zirconium and zirconium base alloys, it is necessary in processing the metal into strip of commercial quality to remove oxides, Scale and the like therefrom. Attempts have been made in the past to effectively remove the oxides from such metal strip by continuously passing the strip through a heated alkali metal caustic bath formed of molten salts such as the commercially available Virgo and Kolene salts, and thereafter pickling the treated strip in a suitable acid bath. While such procedure has been effective in the production of stainless steel strip, it has not proven to be successful in the cleaning of metal strip formed of the refractory metals of the class of titanium, titanium base alloys, zirconium and zirconium base alloys in that although the oxides are removed, the resulting cleaned strip is so severely burned or electrolytically pitted that a large amount of rejected strip material is produced. This problem has been present in the industry for a considerable period of time and in fact, ever since strip material of the refractory metals has been produced commercially, and while many attempts have been made heretofore to eliminate such electrolytic pitting, such attempts have been without success.

An object of this invention is to provide for removing oxides from the surfaces of strip formed from refractory metals of the class of titanium, titanium base alloys, zirconium and zirconium base alloys while preventing electrolytic pitting of the cleaned strip.

Another object of this invention is to provide a method of removing oxides from the surface of strip formed from refractory metal of the class of titanium, titanium base alloys7 zirconium and zirconium base alloys by immersing the strip in a heated caustic bath and maintaining the strip immersed by means of at least one rotatable iron-base roll, there being a difference in potential between the iron-base tank containing the caustic bath and the strip metal immersed therein with the strip metal having a negative polarity with respect to the tank, and for effecting a ow of current between the tank and the roll to develop a difference in potential therebetween substantially equal to the difference in potential between the tank and the strip whereby the roll is maintained at the same potential and polarity with respect to the tank as the strip to decrease current flow between the roll and the strip to prevent electrolytic pitting of the strip.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing in which:

Figure l is a view in section of a cleaning tank employed in practicing this invention;

Fig. 2 is a schematic diagram illustrating connections and equipment embodying the teachings of this invention,

and

will readily react with the oxides normally contained on the surface of the strip l0 as the strip is admitted to the Fig. 3 is a schematic diagram of an equivalent circuit u representing the electrical conditions encountered in the use of a tank such as is shown in Fig. 1, only one roll being shown in the equivalent circuit.

Referring to the drawing, and in particular to Fig. 1 thereof, this invention is illustrated by reference to the immersion of a strip 10 of refractory metal of the class of titanium, titanium base alloys, zirconium and zirconium base alloys in a heated alkali caustic metal bath 12 usually having a sodium base and which is in a molten state, the caustic bath being contained in an iron-base tank 14. Associated with the iron-base tank 14 and usually supported on the edges thereof are a number of rolls formed of an iron base and having at least the lower edges thereof immersed in the heated caustic bath 12. As illustrated, three rolls are shown, rolls 16 and 1S being submersion rolls and roll 20 being a wiper roll, the metal strip 10 being passed beneath the rolls 16 and 18 and over the roll 2l). Each of the rolls 16, ll8 and 20 is suitably mounted on shafts 22, 24 and 26 which have the ends thereof supported in suitable bearing housings 23,

base and are preferably heated, and in use are preferably i maintained at a temperature in the neighborhood of 875 to 975 F. so that the salt bath is in a molten state and tank beneath the roll 16. While many variations in the composition of the caustic bath are possible, a typical ex-` i ample of a suitable caustic bath is that of the commercially known Virgo salts having about 88.55% sodium hydroxide, about 7.66% sodium nitrate, 1.72% sodium chloride and 0.53% sodium carbonate, with traces of manganese dioxide present. The Kolene salts formed of Valkali metal hydroxides, mtrates and chlorides as described in Patent No. 2,458,661 are also effective for use in removing the oxides from the metal strip 1t).

In accordance with this invention the submersion roll 18 near the exit end of the tank 14 and usually the other submersion roll 16 and the wiper roll 2t) are disposed to be connected in an electrical circuit relation with the tank 14 and to a suitable source of power supply represented by the rectangle 2S in Fig. 2 so as to impart a dierence in potential between the iron-base tank 14 and the ironbase roll 18 as well as the iron-base tank 14 and the ironbase rolls 16 and 20, if desired. Thus the shaft 24 of the roll 18 is provided with a commutator sleeve 30 on which sliding brush contact members 32 ride, the brush contact members 32 being connected through an adjustable resistor 34 and a manually operable switch 36 to conductor 38 which is connected to the negative terminal of the power supply. The tank 14 is connected as through conl the rolls 16 and 20 are also connected as through conductors 60 and 62, respectively, to conductor 43. In practice the tank 14 may be considered as being connectedV to Patented Mar. 1l, 1358 one ofthe rolls .1.6, 18 and'20, each of the shaftsy22, T24

and 26 is provided with a commutator sleeve 64,66 vand 68, respectively, each commutator sleeve having aV sliding contact member 70, 72 and 74, respectively,;in engagement therewith for cooperating therewith inmaki'ngan electrical connection. The brushes 70, 72 and 74 are connectedv as by conductors 76, 78 and 80, respectively, .to xed contact members 82, 84 and 86, respectively.which are disposed toibe engaged by a movable contact, member 88 connected` to one terminal of a voltage meterf90. yThe other terminal of the voltage meter 90 is connected asfby conductor 92 to the tank 14 as at 94.

.Referring to Fig. 3 of the drawingthe reason `rfontlle electricalconnections illustrated in Fig. 2 will become apparent when the equivalent circuit for one of..,the rolls, as for example, roll 18, is considered. InfEig. 3;..the strip is illustrated as having a line or strip resistance RL shown kat `96 and a combined leakage resistance Rs shown at`98 of the strip through the supporting structure to ground. The tank 14 contains the molten caustic bath with the insulatedl roll 18 being -at least partially immersed inthe bath. The roll 18 is connected in series with a resistance RK shown at 100 which representsthe resistancefofv the molten caustic bath between the .tank 14 andthe roll 1S. Likewise, in series with the roll 18 and connected to the strip 10 is a resistance RC shown at 102 which represents the contact resistance of the strip 1'0 as the strip passes beneath the roll 18. In a similar manner it will be appreciated that there is an open vcircuit potential E -between the strip 10 and the iron-basetank 14 caused by a battery action in the bath, such open circuit potential E being represented in Fig. 3 by the battery 1:04 which has an internal resistance RB shown at 106 which'is assumed tofbe of the same magnitude as the bath resistance RK. A slight battery action is also probably present between the roll 18 and strip '10 as illustrated by the battery 108 and the internal resistance RB of the battery as shown at 110 in series connection between theroll 18 and the strip 10.

From measurements (of potential between astrip of titanium base alloy having a thickness of .092 inchand an `average width of about 40 inches and the iron-base tank 14, it was found that a verysmall leakage current between the strip and the different rolls may be present, which leakage current flows `out of the strip to the reel (not shown) and back to `the tank .14 through the s1truc 'I tural members of the assembly. When the strip 10 is cut at the end, a diilerence in potentialV between the tank and the strip at the point of the cut is found which is substantially equal to the `normal lmeasured difference n potential of the tankv 14 andthe stripf10 at thetank.l

For this reason the eitect of theleakage currentscan bel dismissed in the theoreticalconsideration of lthcequivalent circuit.

From actual measurements observed, the resistance RK of the molten caustic bath between the ,insulatedsubl` mersion roll 18 and iron-base tank 14, that is, resistance 100, was found to varyfrom`.005 to .010 ohm-depending uponthe temperatureof the bath, whereas the open circuit vpotential E as represented by the 'battery 1,104

was Y-found to beabout 1.1 volts. The battery resistance RB Hshown -at- 106 may -beconsidered; *as -stated hereinbefore,-to beaofthe Asameemagnitude yas-.the-resistance RKiofl the bath, that is, of wfromt005 .to .'010; ohm. fil Since the .normal current How. in .the strip `:'101ineach.diiection from'. the `ro1l118.` is in sutlicient,` lhaving. been vcomputed:A tof be..only..0.'l08 ampere inneachedirection;,.to.f;cause. electrolytic-burning of thegstrip 10,1thetresistance,RLgothe strip `10, and-,the combined/leakage,.resistance RB of-,the strip t-to, ground can .,be,. ignored., Lin kthe theroetical consideration .of .theek-cuits. ,Likewise the, ,elfect oi. the Qcelll represented by the battery 108 and battery resistance RB shown at 110 between the roll 18 and the strip isso slight that it can also be ignored.

With such assumptions it is possible to calculate the approximate current and voltage necessary to eifect a difference in potential between the tank 14 and the iron base roll 18 which will be substantially equal to the difvference in potential normally found between thetank 14 and the strip 10 immersed in the caustic bath 12 in the tank 14. In effecting actual voltage` measurements between the-tank 14 and the strip V10 prior lto effecting a ow of current between theiron-base tank and theroll 18, it `isjound, thatthere is a difference in potentialpf .4 volt with the strip being of negative polarity with respect to the tank 14. Since the openfcircuit potential E as represented by the battery 104 measures 1.1 volts, then the potential drop across the internal resistance RB of -the..battery `as represented bythe resistance. `1,06 rnust bQthe difference between the open circuit potentialpf 1.1 volts and the actual measured difference in potential of .4.voltor .7.volt. Thus the current ow AI,(total)v from thetank 14 to the vstrip 10 canbecalculated bydividing the potentialv drop across the resistance v106 ,-byI the resistance 106,.,or

f E I totalrfl 0'7 vSince thereare two of .the submersion rolls 16` and 18 in l potential by ,the current at each ysubmersion roll, that is,

Sinceit is known from vactual Vmeasurements..thatthe 1 resistancer RK ofthe caustic bathbetween` thetankand the,roll.18 isfequal to .005ohm, then it is evidentthat the contact resistance RC at 192 is only .0007 ohm.; 4Since the opencircuityoltageE as Vmeasured is 1.1 volts, then thellrilfcnt requiredfor .the two submersion 1,'o`llsy assumipgthat the contact-resistance, Re and :resistance RK f .of the caustic .bathare equally divided between4 the rolls,

can be calculated by dividing-the open.. circuit voltage 1.1 by one-half `of, the l.total resistance RK of the caustic kbath on .0025 for a total of 440 amperes'or. the` equivalent Aof a maximum of 220 arnperes. perroll to impart to the -roll anegative .polarity .with a difference in vpotential bet-weenthe tankv 14 andthe roll substantially equaltofthe differencev in potential between the tank14,and vthe strip 10 immersed ,in the lcaustic lbath ,12.

lIn practice it isfoundthat 4the actualcurrent required f is not quite ashigh -as the current calculated, sincecertain assumptions .tending toward a higher current werey made inthe calculations. Actually where the rol1s-16 and 18 are. well insulated from the` tank, a flow of currentof a valueof about amperes is effective for causing a sufcient dilerence fin potential .between "the .iron-base ltank 14 `and lthe iron-basesubmersion,rolls wheresuch rolls have a dametenof.ltinches and. alengthf, 5 feet,VV to smdecreasethecurrent. flowrbetweenthe 1011 ,endjjth strip .,10., as to.effectively A eliminate A all electrolytic pitting -Jofuhe strpofcourseittheinsulation ofthe-.Kills is not Suicient, .thenmere current, isrequiredgto creat@ thernecessarrpotential. diterenwm.prevent arcilla. be' tween the submersion rollan'd'thestrip 10.

In generation. .thestrip 110,,ofrefratcrrmetal 'of Pre-- determinedthicknessasiat example 059,2 inh ,thifkffand predetermined width, as for example 40 inches vwide', is

assenso fed into the molten caustictbath 12 beneath the `submersion roll 16 and the submersion roll 18 and over the wiper roll 20 and from thence to a suitable acid pickling, annealing and heat treating equipment (not shown). As the strip enters the molten caustic bath 12, the strip is usually coated with oxides, as for example, in the case of titanium or titanium base alloys, a coating of titanium dioxide is present on, although not uniformly distributed over, the surface of the strip. Because of the presence of the film of titanium dioxide on the strip as it enters the molten caustic bath and is held therein by reason of the submersion roll 16, there is normally little current flow between the roll 16 and the strip 10. As the strip continues through the molten caustic bath the molten caustic reacts with the oxides and changes them or removes portions thereof with the result that as the strip comes into contact with the submersion roll 18, an increase in the current flow between the roll 18 and the strip 1G will be effected as well as between the wiper roll 20 and the strip 10. Thus arcing to cause electrolytic pitting of the strip is usually found in the area of the submersion roll 1S and the wiper roll 20, the zones of the higher flow of current, as opposed to the area of contact with the submersion roll 16. In practice with the rolls 16, 18 and 2t) and the tank 14 connected as previously described, the manually operable switch 36 is actuated to a circuit closing position to effect a flow of current from the tank 14 to the roll 18, such current fiow being controlled by adjusting the adjustable resistor 34 in circuit relation therewith so as to create the difference in potential between the roll 18 and the tank 14 necessary to substantially equal the difference in potential between the tank 14 and the strip 10 with the polarity of the roll 18 being negative with respect to the tank 14. The potential difference can be readily determined by moving the contact member 88 to the fixed contact member 84 to obtain a reading on the voltage meter 90 of the potential between the roll 18 and the tank 14. Under such conditions any current ow between the iron-base roll 13 and the strip 10 is so decreased or completely eliminated as to prevent arcing therebetween with the result that electrolytic pitting or burning of the strip 10 is completely eliminated.

In practice, it has been found that usually the connection to effect the flow of current from the tank 14 to the roll 1S in the manner just described is sufficient to prevent the electrolytic pitting of the strip 10. However, to insure the4 complete elimination of the electrolytic pitting, the electrical connections previously described for the rolls 16 and Ztl can be effected by actuating the manually operable switches 54 and 58, respectively, to their circuit establishing positions and controlling the flow of current from the tank 14 to the rolls 16 and 20 as by adjusting the adjustable resistors 52 and 56, respectively, so as to definitely effect a ow of current from the tank 14 to each of the rolls 16 and 20 to a degree effective for creating a difference in potential between the tank 14 and each of the rolls 16 and 20 substantially equal to the difference inpotential between the tank 14 and the strip 10 to thereby prevent any arcing between the rolls 16 and 20 and the strip 1t) in the area of contact of each of the rolls 16 and Ztl with the strip 10. The potential difference between the `tank 14 and each of the rolls 16 and 20 can be readily determined by actuating the movable contact member 88 to selectively engage the fixed contact members 82 and 86, respectively, to thereby obtain a reading on the voltage meter 90 of such potential diferences.

The method of this invention is effective for permitting the reaction between the molten alkali metal caustic bath and the oxides of the strip 10 of refractory metal of the class of titanium, titanium base alloys, zirconium and zirconium base alloys without any accompanying electrolytic pitting or burning previously encountered, with the result that untold savings are obtained in that treated strip material is no longer rejected on the basis of electhis latter cleaning of the rolls are not understood, as`

one would normally expect a build-up on the rolls under such operating conditions. It is possible that the flow of current to the rolls removes the surface protective films or oxides normally present on the iron base rolls which are at a negative polarity with respect to the tank with the result that the caustic more vigorously attacks such v metal and keeps it clean. Regardless of the reason, the fact remains that the rolls are actually cleaned with a measurable amount of removal of metal therefrom with the result that no shutdown time is required for replacing the rolls with clean rolls such as was previously encountered in prior practice. Instead, where the previous systems had to be shut down, usually during or at the end of a 2li-hour run, the present method makes it possible to continuously clean strip supplied to the bath without any shutdowns by reason of changing rolls and without making any corrections to prevent electrolytic pitting. The first installation utilizing the method of this invention was run continuously for six weeks without any strip material cleaned thereby being rejected because of electrolytic pitting or burning and without thenecessity of replacing any of the rolls utilized in themolten caustic bath, the line being shut down after such period beginning of a run and will not need to be adjusted during the run, although as a precautionary measure, readings are usually `taken periodically on the voltage meter 9h to determine if operating conditions are being maintained. As will be understood, with the rolls 16, 1S and 2@ completely insulated from the tank 14, the current required will be maintained at a substantially constant value, and in practice ranges from about to 150 1 amperes depending upon the temperature of the molten caustic bath 12 in the tank 14, for as the temperature increases the current requirements increase.

While the electrical connections have been described with reference to hold-down rolls 16 and 18 and the wiper roll Ztl, as has been indicated hereinbefore, the method of this invention is operative where the electrical connections are maintained for only the hold-down roll 18. It is essential that such electrical connections be maintained at all times to the hold-down roll 18.

Thus, with the switch 36 in the open circuit position and the switch 54 in the circuit establishing position, it is found that the caustic reacts with the film of oxides present on the strip 10 of refractory metal as the strip passes beneath the roll 16 to dissolve, change or remove the oxides from the strip as the strip approaches the roll iii. At this point the difference in potential between the roll 18 and the strip 10 is sumcient to effect a iiow of current therebetween to cause electrolytic pitting or burning of the strip with the result that the strip must necessarily be rejected as unsuitable for commercial applications. 0n the other hand, if the switch 35 is in circuit establishing position and the switch 54 is in an open circuit position, it is found that there is insufficient reaction between the caustic and the film of oxides on the strip 1@ as the strip approaches and passes beneath the roll 16 to sufficiently change or remove the oxides with the result Y the'foxides-is `eected orftheoxides `are removed fromfthe strip, the-method. ofthis invention-'.:mustfbe practiced, although froma practical operational viewpoint` allfthree of'the-Lrollsare actually lconnected in circuit relation to providel the differences@ inV `potential:described. so@ as` to elfectively insure against Acontactfarcingy and electrolytic.

pitting of the strip-inl thel neighborhoodoi any lone of the rollswhich i are partially immersed :in the: caustic bath. p

-Frorn' Hthe description f given' hereinbefore it ywill be apparent that! broadly@this-invention'iis applicabletwherel the lstrip -material. beingwtreated is :off aimetal having a base-dissimilar to .thebase-:metal from .which thev Vrolls 16,18 and; and vthe tank `14.are.formed. `The caustic bathfin ,which the--strip10-=is immersed `\is ineflectau electrolyte which will react .with the oxides present on the surface of the strip. It will therefore be apparent that thisinvention .can be utilized wherev the strip is immersed in-an electrolyteand there isa potential difference normally between the strip and the .rolls and tank of metal dissimilar to the strip. Thus in accordance kwith this invention-a ilow of current between `the itank and the roll olike-metals is effected to produce a `potential difference therebetween which-will ybe .substantially equal. to` the potential diierence` between the tank and ,the strip to. in etfectzdecrease the potentialdifference between the .strip and vthezroll and vthereby minimize ,any/current vflow betweenthe roll andthe. stripY to prevent electrolytic pitting of' the stripas fitcis in the bath ofthe reacting electrolyte.

The method of `this invention is relatively simple and can be readily practiced by ,anyoney skilledV in the art. Standard equipment `is utilized, .the only change over the existing practice being'the necessity of insuring that the hold-downrollsand wiper roll are completely insulated from the tank containingthe caustic bath and that suitable commutator sleeves and sliding contact members are provided: for connecting the roll into circuit relation to the suitable source ofpower.

I` claim:

v1..' In the method'offcleaning oxides from metal strip formed, from titanium, titanium base alloy, zirconium andzzirconiumbase alloy, the steps comprising, immersing; andV passing the metal strip` through a heatedY caustic bath in, an,iron-base tanlctopelectV a reaction` between thepcausticqand theoxides,fmaintaining the Vmetal strip immersed in the heated causticV bath by means of a rotatable hold-downWiron-base roll having a portion thereof immersedin the heated caustic bath, the iron-base roll beingl electrically insulated from lthe iron-base tank, the metal, istrip, andiron-base roll and iron-base tank normal'ly having a difference in potential therebetween with the strip having a polarity that is normally negative with respect to the rOll and the tank, connecting the iron-base roll and iron-base tank to the negative terminal andthe positive terminal, respectively, of a supply of currentfto effect a ow of current therebetween through the-,heated caustic bath to elect a difference in potential therebetween substantially equal tothe `normal difference inf-potential between the strip and iron-base tank and impart to the iron-base roll a polarity that Vis negative with respect to the `tank to decrease current ow between the iron base rolland the metal strip and thereby prevent v electrolytic pitting of the metal strip in the heated' caustic bath.

v2.Inthe method of cleaning oxides from metal strip formedfrom titanium, titanium base alloy, zirconium and zirconium base alloy,.the stepseomprising, immersingand passipgfthemetal strip through a heatedcaustic bath -firran iron-base tankto effect a :reactionwbetween the caustic..and the-oxides,y maintainingthe metalv strip immersed inthe heated caustic bath by meansv of. a rotatable hold-down iron-baseroll' having; as. portionf thereoftfimmersedin ,the .heated .:.causticbath,` the..ir.on-base-` roll beingflelectricallyiinsulatednfronnthe. ,iron-base;v tank, the metal-'strip and iron-baseroll andiron-base tank-normally having :ail dierience; .rinipotential therebetween with- .the strip 'having; afpolarity that ,is ,normally negativer with `respecttathefpolarityfof the rollandfthetank, connecting the iron-base roll and:iron.baseitank;to thenegativesterminal .andegthenpositive terminaLr-respectively, of a supply of current to1efect a Ow of current 1through thefheated caustic oath therebetween,tog-effect a, difference in potential, therebetween substantially: equal to "the normaldifterence,inrpotentialsbetween:thestrip and the `tank and impartatotheiron-baseroll-a polarity of the same orderas ythatiof `the:n1'etal strip.fwith respect to -the viron-base `tank to prevent arcing between the roll=.and,the strip.

3. Inthe methodofeleaning;oxides .from metal strip formedfrorn titanium,titaniumtbase alloy, zirconium and .z1rcomum hase alloy, the-.steps comprising, immersing andy passing the metal,- strip Ithrough a heated caustic bath in-.anl iron-base tank toeiect a reaction between the causticv and/.the oxides,imaintainingy the metal strip submerged `in the heatedbath `by means of' a rotatable hold-down iron-base roll disposed in engagement therewith and-having a portion thereof immersed in the heated caustic bath,the iron-base rollbeing electrically insulated from-the iron-base tank,I the ,metal strip in the heated caustic;- bathI normallyfbeing: at ay potential different from that of they iron-base roll andsiron-base tank, connecting the iron-basev roll and iron-base tank to the negative terminalandthe .positive terminal, respectively, of a supply current toeiect aow"of vcurrent therebetween through :the `heated caustic bath to impart yto the iron base roll apotential different from that of the iron-base tank and. substantially equal` to the, normal potential of the metal'` strip yto thereby substantially,eliminate the difierencetin potential ybetweentthe metal stripandthe iron-base roll and prevent eleetrolytic pitting of the metal stripr in the heated caustiebath.

y4. Inthesmethod of cleaning oxides from metalrstrip formed fromftitanium, titaniumbasealloy, zirconium and zirconiumI base alloy, the steps comprising, .immersing and passing the metal strip through a heated caustic bath in an iron-base tank to eiect a reaction between the caustic and the oxides, maintaining the metal strip submerged inthe heated caustic bath by means of a rotatable hold-down iron-base roll disposed in engagement therewith and having a portion, thereof immersed in the heated ,caustic bath, .the iron-base roll being elec- ,trically .insulatedfrom ythe'iron-base tank, the metal strip in the `heated caustic bath normally beingy at a potential different from .that of they iron-base roll and iron-base tank, connecting the iron-base roll and iron-base tank to the negative terminal and thev positive terminal, respecgtively, of` a supply-of, current` to effect a flow of current therebetween 4through the heated caustic bath to impart to.;the iron-base roll a polarity different from thatof the iron-base tank whereby the; potential between the ironbase tank 'and `the iron-base roll is substantially equal `to the normalpotential between theiron-base tank and the metall striprto thereby substantially eliminateL any ,difference. in1potentialvbetween the. iron-base roll `and the metal stripyand prevent electrolytic pitting of `themetal strip as the caustic reacts with the oxides of the strip `in thevheated causticbath.

5.Inthe :method vof cleaning oxides from metalstrip :formedv from -refractory `metal of the` class of titanium, titanium base alloys, zirconium and zirconium base alloys, the steps comprisingrirnmersing and passingk the metal stripthrough aheatedalkali metalcaustic bathin an ironbase tank to eiecta reactionrbetween the caustic and the oxidesrofthetmetal, strip, maintaining the metal strip immersed in theheated caustic bath by passing thestrip beneath av rotatable hold-down iron-.base roll having .a.-p ortion thereoimrnersed, in theV ,heatedcaus'tic bath,

the iron-base roll being electrically insulated from the iron-base tank, the metal strip and the iron-base roll and iron-base tank normally having a dilerence in potential therebetween with the metal strip having a negative polarity with respect to the polarity of the iron-base tank and iron-base roll, connecting the iron-base roll and ironbase tank to the negative terminal and the positive terminal, respectively, of a supply of current to eiiect a tiow of current therebetween through the heated caustic bath to eiect a difference in potential therebetween substantially equal to the normal difference in potential between the strip and the iron-base tank and impart to the iron-base roll a polarity that is negative with respect to the polarity of the iron-base tank to decrease current ilow between the iron-base roll and the metal strip and thereby prevent electrolytic pitting of the metal strip as the caustic reacts with the oxides of the strip in the heated causic bath.

6. In the method of cleaning oxides from metal strip formed from refractory metal of the class of titanium, titanium base alloys, zirconium and zirconium base alloys, the steps comprising, immersing and passing the metal strip through a molten alkali metal caustic bath in an iron-base tank to effect a reaction between the caustic and the oxides of the metal strip, maintaining the metal strip immersed in the caustic bath by means of a holddown iron-base roll having a portion thereof immersed in the caustic bath, the iron-base roll being electrically insulated from the iron-base tank, the metal strip and iron-base roll and iron-base tank normally having a diterence in potential therebetween with the strip having a polarity in the bath that is negative with respect to the iron-base tank and iron-base roll, connecting the ironbase tank and iron-base roll to the positive terminal and the negative terminal, respectively, of a supply of current to etect a ditterence in potential therebetween substantially equal to the normal diierence in potential between the strip and the tank and impart a polarity to the iron-base roll that is negative with respect to the iron-base tank to thereby effect a decrease in current tow between the iron-base roll and metal strip in the caustic bath and prevent electrolytic pitting of the metal strip as the caustic reacts with the oxides of the strip in the caustic bath.

7. In the method of cleaning oxides from metal strip formed from refractory metal of the class of titanium, titanium base alloys, zirconium and zirconium base alloys, the steps comprising, immersing and passing the metal strip through a molten alkali metal caustic bath in an iron-base tank, controlling the immersion and exit of the metal strip with respect to the caustic bath by means of a plurality of spaced rotatable iron-base rolls having portions thereof immersed in the caustic bath, the iron-base rolls being electrically insulated from the iron-base tank,

the metal strip and the iron-base tank and iron-base rolls normally having a difference in potential therebetween with the metal strip having a polarity that is negative with respect to the iron-base tank and iron-base rolls, connecting the ironbase tank and each of the iron-base rolls to the positive terminal and the negative terminal, respectively, of a supply of current to effect a ow of current through the caustic bath between the iron-base tank and each of the iron-base rolls to produce a difference in potential therebetween substantially equal to the normal diterence in potential between the strip and iron-base tank and impart a polarity to each of the ironbase rolls that is negative with respect to the ironbase tank whereby current tiow between each of the iron-base rolls and the metal strip is decreased to prevent electrolytic pitting of the metal strip in the caustic bath.

8. In the method of cleaning oxides from metal strip formed from titanium, titanium base alloy, zirconium and zirconium base alloy, the steps comprising, immersing and passing the metal strip through an electrolyte which will react with the oxides of the strip, the electrolyte being contained in a tank formed of metal dissimilar with respect to the metal of the strip, maintaining the metal strip immersed in the electrolyte by means ot a rotatable hold-down roll formed of a metal having the same base as that of the metal of the tank, and having at least a portion thereof immersed in the electrolyte, the roll being electrically insulated from the metal of the tank, the metal strip and metal roll and metal tank normally having a potential difference therebetween, connecting the roll and tank to the negative terminal and the positive terminal, respectively, `of a supply of current to etect a ow of current therebetween through the electrolyte to impart a potential dierence therebetween substantially equal to the potential between the strip and tank and a polarity to the roll of the same sign as the strip is with respect to the tank to thereby decrease the potential dierence between the strip and roll and decrease the current ow between the roll and the strip to prevent. electrolytic pitting of the metal strip in the reacting electrolyte.

References Cited in the iile of this patent UNiTED STATES PATENTS 1,457,688 Bohnhardt June 5, 1923 2,503,217 Prust Apr. 4, 1950 2,738,294 Spence Mar. 13, 1956 2,778,791 Duncan Ian. 22, 1957 2,780,594 Dailey Feb. 5, 1957 OTHER REFERENCES Metal1urgia, vol. 49, No. 295, May 1954, page 256, by Durkin. 

4. IN THE METHOD OF CLEANING OXIDES FROM METAL STRIP FORMED FROM TITANIUM, TITANIUM BASE ALLOY, ZIRCONIUM AND ZIRCONIUM BASE ALLOY, THE STEPS COMPRISING, IMMERSING AND PASSING THE METAL STRIP THROUGH A HEATED CAUSTIC BATH IN AN IRON-BASE TANK TO EFFECT A REACTION BETWEEN THE ACUSTIC AND THE OXIDES, MAINTAINING THE METAL STRIP SUBMERGED IN THE HEATED CAUSTIC BATH BY MEANS OF A ROTATABLE HOLD-DOWN IRON-BASE ROLL DISPOSED IN ENGAGEMENT THEREWITH AND HAVING A PORTION THEREOF IMMERSED IN THE HEATED CAUSTIC BATH, THE IRON-BASE ROLL BEING ELECTRICALLY INSULATED FROM THE IRON-BASE TANK, THE METAL STRIP IN THE HEATED CAUSTIC BATH NORMALLY BEING AT POTENTIAL DIFFERENT FROM THAT OF THE IRON-BASE ROLL AND IRON-BASE TANK, CONNECTING THE IRON-BASE ROLL AND IRON-BASE TANK, CONNECTING THE IRON-BASE ROLL AND IRON-BASE TANK TO THE NEGATIVE TERMIKNAL AND THE POSITIVE TERMINAL, RESPECTIVELY, OF A SUPPLY OF CURRENT TO EFFECT A FLOW OF CURRENT THEREBETWEEN THROUGH A HEATED CAUSTIC BATH TO IMPART IRON-BASE ROLL A POLARITY DIFFERENT FROM THAT OF THE BASE TANK AND THE IRON-BASE ROLL IS SUBSTANTIALLY EQUAL TO THE NORMAL POTENTIAL BETWEEN THE IRON-BASE TANK AND THE METAL STRIP TO THEREBY SUBSTANTIALLY ELIMINATE ANY DIFFERENCE IN POTENTIAL BETWEEN THE IRON-BASE ROLL AND THE METAL STRIP AND PREVENT ELECTROYLIC PITTING OF THE METAL STRIP AS THE CAUSTIC REACTS WITH THE OXIDES OF THE STRIP IN THE HEATED CAUSTIC BATH. 